The invention relates to capacitive control panels. More particularly the invention relates to capacitive control panels incorporating touch sensitive and force sensitive input means.
There is an increasing demand for robust and aesthetically pleasing control panels (user interfaces) for controlling devices. Over recent years capacitive sensing techniques have become well established and accepted in this field. Common examples of devices that include capacitive control panels are touch-sensitive display screens and touch-sensitive keyboards/keypads, e.g., as used for controlling consumer electronic devices/domestic appliances.
It is known for capacitive control panels to include both touch sensitive inputs (e.g., capacitive position sensors) and force sensitive inputs (e.g., conventional push buttons/switches). For example, versions of the “iPod mini” manufactured by Apple Computer Inc. have a touch sensitive scroll wheel overlaying a number of mechanical switches.
FIG. 1 schematically shows a section view of a control panel 2 of this general type. The control panel 2 is mounted in a wall 4 of a device to be controlled. The control panel includes a capacitive touch sensing element 6 in the form of a ring perpendicular to the plane of FIG. 1, and a number of conventional mechanical switches 8. Two of the mechanical switches 8 are apparent in the cross-section view of FIG. 1. The capacitive sensor 6 and mechanical switches 8 are coupled to appropriate control circuitry (not shown).
The capacitive position sensing element 6 is formed on a printed circuit board (PCB) acting as a structural platform 10. The platform PCB 10 and the capacitive sensing element 6 are covered by an outer protective layer 14. The platform PCB 10 is tiltably mounted on a central support 12 so that it can move within an opening in the wall 4 of the device. The support 12 is attached to a base PCB 16. The base PCB 16 and the wall 4 are fixed together. The position of a user's finger touching the sensing element 6 is determined by the capacitive sensor control circuitry and used to control the device accordingly.
The mechanical switches 8 are mounted on the base PCB 16 beneath the capacitive position sensing element 6. Each mechanical switch 8 comprises a deformable diaphragm 8B disposed over a central electrode 8A. Each diaphragm extends away from the base PCB 16 to a height at which it just touches the underside of the platform PCB 10. Switching action is achieved by deforming a selected diaphragm so that it contacts the central electrode 8A. This is done by pressing down on the capacitive position sensing element above the desired switch. This causes the platform PCB 10 to tilt about its central support 12 and compress the diaphragm of the selected switch to bring it into contact with its central electrode.
A user may thus provide control instructions through appropriate use of the capacitive position sensing element 6 and the mechanical switches 8 in accordance with the controlled device's means of operation.
The control panel 2 shown in FIG. 1 provides a compact and intuitive user interface, but has a number of shortcomings. For example, the use of conventional push switches means that the control panel only has binary sensitivity to mechanical force input. That is to say the control panel can only indicate whether or not a switch is open or closed. There is no analogue sensitivity to the magnitude of force applied. This restricts the flexibility of the control panel to respond in different ways to different forces. Furthermore, the overall structure is relatively complex and the use of two different sensing techniques (i.e., conventional switches for mechanical force sensing and capacitive sensing techniques for touch sensing) means the required complexity of the control circuitry is increased since it has to be able to accommodate both types of sensor.
Force sensors based on capacitive sensing techniques are known. FIGS. 2A and 2B schematically section views of one such capacitive force sensor 20. The force sensor 20 is mounted on a base 22 and comprises a lower electrode 26 and an upper electrode 28. The lower and upper electrodes are separated by a compressible dielectric material 24 and connected to capacitance measurement circuit 21. The capacitance between the lower and upper electrodes 26, 28 as measured by the capacitance measurement circuit 21 depends on the magnitude of their separation.
FIG. 2A shows the force sensor 20 in a relaxed state (no force applied) and FIG. 2B shows the force sensor 20 with a perpendicular force F applied. The force compresses the dielectric material 24, and thus brings the lower and upper electrodes 26, 28 closer together. This registers as a change in their mutual capacitance measured by the capacitance measurement circuit 21. The extent to which the dielectric material 24 is compressed (and hence the change in separation between the electrodes 26, 28) depends on the magnitude of the force F. Accordingly, the output from the capacitance measurement circuit 21 provides a measurement of the force F applied.
Sensors of the kind shown in FIGS. 2A and 2B may be used in various ways, for example in feedback circuits of robotic gripping hands, or as air pressure monitors in automobile tires. However, a problem with incorporating such force sensors into control panels/user interfaces is that as well as being sensitive to directly applied forces, such sensors will also be sensitive to changes in capacitance caused by nearby pointing objects, such as a user's finger, even when not pressing on the sensor (in the same way that conventional capacitive touch sensors respond to proximate objects).